Geopuzzle (updated with answer)
Haven’t done one of these in a while.
This could be easy, might be difficult … I’m not sure — simply comment below explaining this photograph.
If you happened to be there with me you are not allowed to answer :)
UPDATE: This was a tough one. I was on a field trip last weekend and as soon as I saw this feature I instantly knew I’d make it a great geopuzzle.
A few of the commenters speculated about this being some kind of collapse feature. That is exactly what it is — but the part of the story that you might not have guessed is that the collapse happened since the 1860s. This is from the Black Diamond coal mine (inactive since the early 1900s) near Antioch, California. The coal seam was removed and the overlying sedimentary rocks collapsed into the space. The dark material is bits of pieces of the lignite coal left.
Even though I knew what this was when we came across it (as our guide pointed it out), I was amazed how similar it looked to other soft-sediment or local-collapse deformation features I’ve seen from the geologic record.
To see more photos of this mine, check out my Flickr set here.
Thanks for playing!